1. Field of the Invention
This invention relates to tube mills of the type having grinding bodies for dry grinding granular materials and the like.
2. Description of the Prior Art
Present day grinding mills -- particularly those which are used for dry grinding coarse materials such as cement clinker -- incorporate a short compartment to improve the performance of the mill. The short compartment is generally installed in order to maintain a permanent pool of ground material in the upstream grinding compartment.
The short compartment is devoid of any lifters or other devices that would prevent the formation and maintenance in it of a pool of fluidized material of substantial depth. However, it may have a central part designed for discharging material into the downstream grinding compartment or out of the mill. The central part may have scoops the ends of which are at such distance from the peripheral wall of the mill as to allow the pool to remain at the desired level.
When present in the upstream grinding compartment, the pool of fluidized material reduced considerably the wear on the mill lining and the grinding bodies. Further, there is an increase in the efficiency as measured in the power consumption. In addition, the usual grinding noise is reduced and the grinding process can be carried on at a grinding noise level lower than was previously considered desirable when an optimum of the grinding efficiency should be achieved.
The size of the central opening through the dam ring -- and to some extent the central scoop apparatus -- constitute a factor which determines the actual depth of the fluidized pool in the first or sole grinding compartment. In certain circumstances this condition might be disadvantageous.
The size of the central opening is often fixed by the preferred depth of the charge in the downstream grinding compartment or the discharge outlet through a hollow trunnion and dependent upon the required volume of ventilating air to be passed through the mill. This presents an additional difficulty in designing the central opening of the dam ring to suit the conditions aimed at in the first or sole grinding compartment.
Further, in a multi-compartment mill in which the short compartment separates two grinding compartments the central opening of the dam ring may give rise to some difficulties in the performance of the mill because grinding bodies from the downstream grinding compartment may, in their tumbling action during the rotation of the mill, pass to the short compartment. In the known mills the grinding bodies cannot be returned to the grinding compartment because the short lifters forming the discharge apparatus are unable to pick up the grinding bodies.
In an attempt to prevent grinding bodies from passing back from the downstream grinding compartment the dam ring might be provided with a coarse screen or a shroud protruding into the downstream grinding compartment to serve as an obstacle to this undesirable passage of grinding bodies. However, in certain circumstances, for example, when using very small grinding bodies in the downstream grinding compartment, it might still be difficult to prevent the grinding bodies from passing into the short compartment and yet ensure the unhindered passage of the ground material and ventilating air through the mill.
I have invented a grinding mill in which these disadvantages are eliminated and in which efficient transporting of the ground material is obtained.